Process and apparatus for lining conductive tubes with insulating material

ABSTRACT

Process and apparatus for producing layer of insulating material on interior of a conductive tube in which the insulating material in a fluid carrier is sprayed interiorly of the tube by a spray and carrier evacuating head as the tube is extruded around said head and advanced.

United States Patent Longoni et al.

[ Feb. 22, 1972 [54] PROCESS AND APPARATUS FOR LINING CONDUCTIVE TUBES WITH INSULATING MATERIAL Sergio Longoni, Milan; Antonio Portinari, Sesto San Giovanni (Milan), both of Italy Industrie Pirelli S.p.A.

Mar. 11, 1970 Inventors:

Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data Apr. 5, 1969 Italy ..15l48 N69 US. Cl. ..l64/76, 18/14 H, 118/317, 164/85, 164/268, 164/270 Int. Cl ..B22d 11/12 Field of Search ..164/76, 85, 86, 91, 268, 267, 164/270; 29/5274, 600; 18/14 H; 118/317; 138/97, 141, 145, 146

[56] References Cited UNITED STATES PATENTS 3,422,795 1/l969 Smith ..1 18/317 X 3,022,552 2/1962 Tessmann ..164/268 1,730,638 10/1929 Young ..18/14 H X FOREIGN PATENTS OR APPLICATIONS 993,277 10/1951 France ..18/l4 H Primary Examiner-J Spencer Overholser Assistant Examiner-John E. Roethcl Anomey Brooks, Haidt and Hutfner [5 7] ABSTRACT Process and apparatus for producing layer of insulating material on interior of a conductive tube in which the insulating material in a fluid carrier is sprayed interiorly of the tube by a spray and carrier evacuating head as the tube is extruded around said head and advanced.

13 Claims, 1 Drawing Figure PROCESS AND APPARATUS FOR LINING CONDUCTIVE TUBES WITH INSULATING MATERIAL The present invention relates to a process for lining conductive tubes obtained by extrusion with insulating material, as well as to the apparatus for carrying out said process.

The process and the apparatus according to the invention are of particular interest in the application of insulation to the inner surface of aluminum waveguides.

The attempts heretofore made to solve, in a technically efficient way, the inner lining of aluminum tubes with insulating materials have proved unsatisfactory because of the serious difficulty in obtaining an intimate adhesion between the insulating and the inner metallic surface of the tube. The methods used up to the present time, which have not had a wide industrial development, consist primarily in preparing separately an aluminum tube and a lining of insulating material, for instance polyethylene, which has an outer configuration as close as possible to the inner configuration of the aluminum tube. This lining, the thickness of which is of the order of tenths of mi]- limeters, is inserted in collapsed condition inside the tube and is then inflated in such a way as to cause it to adhere to the tube by pressure.

As has been experimentally ascertained, difficulties are encountered in ensuring perfect adherence between the lining and the tube, as air remains easily trapped between them.

The present invention aims at eliminating the above-indicated difficulties by providing a process for carrying out an inner lining of the surfaces of conductive tubes, in particular of aluminum waveguides, which provides perfect adhesion of the insulation to the metal and a uniform thickness of the lining without any break in the continuity of the bond therebetween and thereby ensures a good insulating action at all points.

A further object of the invention is that of providing, when necessary, an insulating inner lining, the thickness of which may be considerably smaller than one-tenth of a millimeter and hence, considerably thinner than prior art linings.

In particular, the object of the present invention is to provide a process for the inner lining of conductive tubes obtained by extrusion, such as aluminum waveguides, according to which appropriately selected insulating particles, electrostatically charged and contained in a conveying fluid, are pushed or sprayed uniformly, as the tube comes out of the extruder head, on the inner surface of the tube itself as it advances at high temperature. The particles are sprayed on the mechanically stabilized portion of the tube which is the nearest to the extrusion head, in order that they may adhere perfectly to said surface and may consequently form an insulating layer devoid of breaks in continuity, an appropriate suction device compelling the conveying fluid to flow out of the tube after the deposition of the insulating particles.

A further object of the present invention is to provide an apparatus for carrying out the process according to the invention, which comprises an extruder, in which the inner mould of the head is provided with at least means for spraying the conveying fluid containing the insulating particles or powders on the inner surface of the tube obtained by extrusion, and with at least a means able to suck out or remove the conveying fluid after the insulating particles have been deposited on said surface.

The single FIGURE of the attached sheet of drawing illustrates by way of example and in a schematic, cross-sectional, side elevation view a practical embodiment of the apparatus for carrying out the process of the invention. Said figure shows diagrammatically, in longitudinal section, the head of an extruder provided with the means for spraying the insulating particles on the inner surface of the tube to be extruded, and with suction means for removing the conveying fluid from the inside of the extruded tube after the deposition of the particles conveyed by said fluid.

ln the drawing, the reference numeral 1 indicates the extrusion head which comprises an outer mould 4 and an inner mould 5 which define a duct for conveying the metal 2, in melted state, towards the outlet 3.

The inner mould 5 is provided with a lance 6, coaxial with it. An element 7 of insulating material is inserted between the inner mould 5 and the lance 6, and permits the maintaining of the potential of the extruded tube 12 at a value different from the potential of the lance 6. The lance 6 contains the following elements, coaxial with it: a nozzle 8, provided with an adhesive spray duct 13 of annular section, connected to a compressor (not shown) and a duct 10, also of annular section, connected to a vacuum or suction means (not shown) and provided with a suction opening 14.

A cooling chamber 11, having an annular section, is provided between duct 10 and duct 13. The chamber is coaxial to them and is able to maintain duct 13 at a lower temperature compared to the parts exterior thereto. The cooling chamber 11 may also be divided into two intercommunicating halfchambers by means of a longitudinal partition.

Molten metal such as aluminum, contained in the extruders head is forced through the duct defined by the outer mould 4 and by the inner mould 5, and out ofthe outlet 3, where a tube 12, for instance a waveguide, begins to be formed. The lance 6 is disposed in such a way that the nozzle 8 opens in proximity of the zone of the tube extrusion and in a position at which the tube is still hot, but its dimensions are stabilized.

As the tube 12 is formed, particles of insulating material are sprayed in the form of a mist on the inner surface of the tube 12 by the nozzle 8 and a potential difference is maintained between the tube 12 and the lance 6 in any manner conventional in electrostatic spray coating, such as by connecting a direct current source between the tube 12 and the lance 6, so as to aid in causing such particles of insulating material flowing out of the duct 13 along the paths 9 to become deposited on the inner surface of the tube 12. The particles adhere to the inner surface and, depending on the material employed, may melt thereon, to form a uniform insulating layer 15 which is continuous and may be as thin as 0.1 mm. or less. It will be apparent that the thickness of the layer depends upon the material sprayed, the rate at which it is sprayed and the speed at which the tube 12 is formed and is drawn away from the extruding head.

The material sprayed may be of any of many different insulating materials known in the art and may, for example, be polyethylene or polypropylene in a gaseous or liquid solvent or carrierv therefor. One particularly suitable material is polyethylene having a density of about 091 and an average molecular weight of 1,500. Suitable gaseous carriers or solvents are nitrogen or dichlorodifluoromethane and suitable liquid is tetrachloroethylene. As examples of preferred compositions, a mixture of 5 percent by weight of polyethylene or polypropylene in tetrachloroethylene may be used.

The converging fluid or carrier, after having deposited the insulating particles, is sucked through the discharge duct 10 and is discharged in a collecting tank or to the atmosphere, depending on whether it is a solvent or a gas and whether the recovery of which is necessary or convenient, or if it merely is air. The fluid may also be passed through conventional purification or recovery systems for reuse.

Cooling fluid, such as water, is circulated in the cooling chamber 11 to maintain the duct 13 at the desired temperature, even though it is disposed in the zone of high temperature of the extruder head.

It is understood that the details of the process according to the invention or the details of construction of the apparatus for carrying out said process can vary according to the needs. In particular, the insulation can be that suitable in view of the particular performance required for the waveguide to be extruded, and the conveying fluid can be chosen among the liquid solvents or the gases which prove to be the most appropriate for the type of insulation, for the working temperatures or for other elements which may affect forming of the layer 15 or the characteristics of the end product. The extruders head can have a configuration different from that described, the nozzle for spraying the fluid containing the charged particles can have a shape selected to provide the desired spray as a function of the insulating material, the fluid employed, the dimensions and shape of the tube 12 and the forward speed of the tube 12.

Also, the shape of the sucking duct can be different from that indicated. For example, it can be annular or concentric with the axis of the inner mold. The position of the spraying nozzle and of the suction opening can differ from those illustrated. For example, they could be reversed with respect to those described and hence, the spraying nozzle could be in a more peripheral position while the suction opening could be in a more central position.

What is claimed is:

l. A process for lining the interior of a conductive metal tube with a relatively thin layer of insulating material of uniform thickness comprising extruding metal of said tube heated to a temperature above the melting point of said insulating material in tubular form and advancing the so-formed tube as it is formed, applying said insulating material in a fluid carrier to the interior surface of said tube as it is so formed and advanced and at circumferential portion on said surface spaced from the point of extrusion and that has stabilized in form but which is at a temperature above the melting point of said insulating material by projecting the insulating material in said carrier radially toward said portion from within said formed tube and in a spray pattern uniformly distributed around the axis of said tube but spaced therefrom and evacuating the fluid carrier remaining after the insulating material has been deposited on said surface from within said tube.

2. A process as set forth in claim 1 wherein said insulating material is in the form of particles which are mixed in a fluid carrier.

3. A process as set forth in claim 1 wherein said carrier is a gas under pressure.

4. A process as set forth in claim 3 wherein said gas is selected from the group consisting of nitrogen and dichlorodifluoromethane.

5. A process as set forth in claim 1 wherein said carrier is a liquid.

6. A process as set forth in claim 5 wherein said liquid is tetrachloroethylene.

7. A process as set forth in claim 1 wherein said insulating material is selected from the group consisting of polyethylene and polypropylene and wherein said carrier is selected from the group consisting of nitrogen, dichlorodifluoromethane and tetrachloroethylene.

8. A process as set forth in claim 1 wherein an electrostatic field is provided between said insulating material and said interior surface for directing said insulating material on said surface.

9. A process as set forth in claim 1 wherein the insulating material and its rate of application and the speed of advance of said tube are selected to produce an insulating layer on said surface less than 0.1 millimeter in thickness.

10. Apparatus for manufacturing a conductive tube having a relatively thin interior lining of insulating material of uniform thickness comprising an extruder for extruding metal heated to a temperature above the melting point of said insulating material and having an extrusion outlet in the shape of said tube for forming said tube, spray means within said extruder and extending beyond said extrusion outlet in the direction of extrusion for spraying said insulating material in a fluid carrier on the interior surface of said tube and as it is extruded, said spray means comprising an annular outlet spaced from said extrusion outlet in said direction and disposed adjacent said interior surface of said tube for directing said insulating material in said carrier radially toward said interior surface, said annular outlet extending around but being spaced from the axis of said tube, and a passageway extending through said extruder and communicating with said annular outlet for supplying said insulating material in said carrier to said annular outlet, and evacuating means within said extruder and extending beyond said extrusion outlet in said direction for evacuating said carrier after said insulating material has been deposited on said interior surface, said evacuatm g means having an opening spaced from said extrusion outlet in said direction and from said annular outlet and having a passageway extending through said extruder and communicating with said opening for withdrawing said carrier from within said tube.

11. Apparatus as set forth in claim 10, wherein said spray means comprises a lance mounted coaxially with said extrusion outlet and having said annular outlet spaced from and coaxial with said extrusion outlet and said apparatus comprises thermal and electrical insulating means mounted between said extruder and said lance for insulating said lance from said extruder.

12. Apparatus as set forth in claim 11, wherein said evacuating means is coaxial with said lance and said opening thereof is positioned intermediate said extrusion outlet and said annular outlet.

13. Apparatus as set forth in claim 11, wherein said passageway of said spray means and said passageway of said evacuating means are formed by a pair of coaxial ducts, a pair of walls of said ducts being spaced from each other and forming a cooling chamber therebetween for the circulation of cooling fluid between said ducts.

nun-u 

2. A process as set forth in claim 1 wherein said insulating material is in the form of particles which are mixed in a fluid carrier.
 3. A process as set forth in claim 1 wherein said carrier is a gas under pressure.
 4. A process as set forth in claim 3 wherein said gas is selected from the group consisting of nitrogen and dichlorodifluoromethane.
 5. A process as set forth in claim 1 wherein said carrier is a liquid.
 6. A process as set forth in claim 5 wherein said liquid is tetrachloroethylene.
 7. A process as set forth in claim 1 wherein said insulating material is selected from the group consisting of polyethylene and polypropylene and wherein said carrier is selected from the group consisting of nitrogen, dichlorodifluoromethane and tetrachloroethylene.
 8. A process as set forth in claim 1 wherein an electrostatic field is provided between said insulating material and said interior surface for directing said inSulating material on said surface.
 9. A process as set forth in claim 1 wherein the insulating material and its rate of application and the speed of advance of said tube are selected to produce an insulating layer on said surface less than 0.1 millimeter in thickness.
 10. Apparatus for manufacturing a conductive tube having a relatively thin interior lining of insulating material of uniform thickness comprising an extruder for extruding metal heated to a temperature above the melting point of said insulating material and having an extrusion outlet in the shape of said tube for forming said tube, spray means within said extruder and extending beyond said extrusion outlet in the direction of extrusion for spraying said insulating material in a fluid carrier on the interior surface of said tube and as it is extruded, said spray means comprising an annular outlet spaced from said extrusion outlet in said direction and disposed adjacent said interior surface of said tube for directing said insulating material in said carrier radially toward said interior surface, said annular outlet extending around but being spaced from the axis of said tube, and a passageway extending through said extruder and communicating with said annular outlet for supplying said insulating material in said carrier to said annular outlet, and evacuating means within said extruder and extending beyond said extrusion outlet in said direction for evacuating said carrier after said insulating material has been deposited on said interior surface, said evacuating means having an opening spaced from said extrusion outlet in said direction and from said annular outlet and having a passageway extending through said extruder and communicating with said opening for withdrawing said carrier from within said tube.
 11. Apparatus as set forth in claim 10, wherein said spray means comprises a lance mounted coaxially with said extrusion outlet and having said annular outlet spaced from and coaxial with said extrusion outlet and said apparatus comprises thermal and electrical insulating means mounted between said extruder and said lance for insulating said lance from said extruder.
 12. Apparatus as set forth in claim 11, wherein said evacuating means is coaxial with said lance and said opening thereof is positioned intermediate said extrusion outlet and said annular outlet.
 13. Apparatus as set forth in claim 11, wherein said passageway of said spray means and said passageway of said evacuating means are formed by a pair of coaxial ducts, a pair of walls of said ducts being spaced from each other and forming a cooling chamber therebetween for the circulation of cooling fluid between said ducts. 